An AC TFEL device is a laminar structure comprising an electroluminescent phosphor material sandwiched by insulators and sets of front and rear electrodes. Typically, the insulators and electrodes which are positioned between the viewer and the phosphor are transparent. Thus, the laminar structure may be fabricated either on an opaque substrate and covered by a glass panel for film-side viewing or the structure may be fabricated directly on a transparent substrate for substrate-side viewing. It has long been a desirable object to provide a full color TFEL device, but to do so requires the generation of light of the three primary colors, red, green and blue.
Blue light is the most difficult of the primary colors to produce. For more than a decade research groups have searched for a phosphor that produces blue light having the desired chromaticity and sufficient luminescence to produce an efficient blue light-emitting TFEL structure. An early type of such a structure is shown in Barrow et al. U.S. Pat. No. 4,751,427. In the M. H. Barrow patent a blue light-emitting phosphor comprising strontium sulfide doped with cerium fluoride is sandwiched by a pair of zinc sulfide layers which are characterized as carrier injection layers. Unfortunately, strontium sulfide is hygroscopic and chemically unstable.
A considerable improvement over the aforementioned cerium-doped strontium sulfide device include an alkaline earth thiogallate phosphor doped with cerium to provide a blue light emitting center. A TFEL structure employing such a thiogallate phosphor is described in Sun et al U.S. Pat. No. 5,309,070. The M. H. alkaline earth thiogallate typically is deposited as an amorphous film which must be annealed to produce a crystalline phosphor. The crystalline structure of the thiogallate is improved by annealing the thiogallate in contact with a ZnS layer. The ZnS layer was originally thought to provide a carrier injection function, but more recently it has been realized that its real function is to act as a nucleation enhancer to help transform the amorphous thiogallate into crystalline form.
Investigators have examined the possibility of providing charge injection layers for other AC TFEL devices, particularly those employing a conventional zinc sulfide phosphor doped with manganese. For example, Kobayashi et al., in IEEE Transactions on Electron Devices 29, 1626 (1982), and Ohwaki et al., in Electrochemical Soc. 137, 340 (1990), investigated, respectively, the use of germanium and silicon dioxide as charge injection layers. Still later, Britton et al., in Proc. of the Sixth Int'l Workshop on Electroluminescence, 286-91 (1992), investigated using thin layers of aluminum to modify the insulator-phosphor interface to promote charge injection into a ZnS:Mn system. These studies produced mixed results. Britton and Ohwaki demonstrated increased brightness of the devices employing the charge injection layer over the respective study's control devices. However, Kobayashi reported an increase in conduction charge, but a decrease in the brightness due to a reduction in the local electric field in the devices with the germanium injection layer compared to the control devices. Thus, with respect to these devices, researchers have been unable to confirm the efficacy of purported charge injection layers.
A considerable improvement in the performance of thiogallate phosphors has now been made possible by the present invention. According to the invention a thin film electroluminescent structure for emitting light in response to the application of an electric field includes first and second electrode layers sandwiching a TFEL stack including at least one insulator layer and a three layer laminate structure. The three layer laminate structure includes a phosphor layer containing an alkaline earth thiogallate, a nucleating layer and an injection layer. The nucleating layer lies between the phosphor layer and the injection layer. The injection layer provides a charge injection function through the nucleating layer for the thiogallate phosphor layer which is of high crystallinity at its interface with the nucleating layer. A preferred injection layer includes indium, for example as the metal or as indium tin oxide. The best material for the nucleating layer is zinc sulfide.